It is possible to obtain the so-called indicative pressure signal representative of the inner pressure of the combustion chamber of an internal combustion engine by providing a pressure sensor such as a piezo-electric element in a bore formed through a member forming the combustion chamber of the engine such as a cylinder head. A pressure gauge may be otherwise interposed between the cylinder head and the cylinder block of the engine, which functions as the pressure sensor for producing the indicative pressure signal.
It will be seen that the internal pressure in the combustion chamber under operation of the engine changes as indicated by a curve A in FIG. 1. When the ignition system of the engine is triggered at an ignition angle .theta. IG, the air-fuel mixture supplied thereto starts firing with a time delay of .theta. d and, subsequently, the internal pressure rapidly increases up to a maximum pressure peak (referred to as an indicative pressure peak hereinafter) and then decreases.
It is known that a crank angle position of the indicative pressure peak has a certain relationship with the state of the engine at which the maximum output is produced, and the indicative pressure peak giving the maximum engine output has been found, by experiment, to be located between 12 to 13 degrees after the top dead center (referred to as ATDC hereinafter) as shown in the drawings. Therefore, ATDC 12 to 13 degrees may be considered as an ideal crank angle region. It is therefore desirable to determine the ignition timing .theta. IG so that the indicative pressure peak occurs within the ideal crank angle region which is ATDC 12 to 13 degrees.
Even if, however, the ignition timing .theta. IG is determined, the indicative pressure peak varies from time to time depending on the operating conditions of the engine and, therefore, an ignition timing control device is desired which can keep the indicative pressure peak within the optimum region.
A feed-back ignition timing control system was disclosed in U.S. Pat. No. 4,48l,925 issued Nov. 13, 1984. The feed-back ignition timing control system controls the ignition timing of an internal combustion engine in response to the indicative pressure signal to keep the indicative pressure peak position within an optimum region. In this prior art system, contamination of the indicative pressure signal by high frequency noises is ignored by providing a gating function for taking the indicative peak information only during a predetermined crank angle region or zone. The particular crank angle zone is defined by timing pulses generated by a pulse generator including a toothed wheel and a pickup for producing a timing pulse at each time a tooth passes before it. The teeth are mounted on the periphery of the wheel equidistantly such as 60 degrees.
However, it has been a problem that such gating function defined by the timing pulses cannot supply the required freedom in view of the wide range of possible applications to various types of engines. In some cases, it has been required to more precisely define the crank angle zone so as to avoid influence by any possible noises in the indicative pressure signal and to obtain a sufficient calculation period for the desired ignition timing for the succeeding engine cycle.